I think this must be from a blue whale. The only scientific paper I have on blue whale is from 1896 by Beneden and is in french. I have just done a bit of google translate, there are some measurements of the bones in the text and they would seem to be about the same size as this lumbar vertebra.

Wow that's exciting! (or is it? Are these very common or is it of some significant interest?). Is it possible for me to gain a copy of the report you have? We have French speakers in our household and it would be great to hear what was documented! Thanks for the help! Michael